The Morphology of Block Copolymer Micelles in ... - IUCr Journals

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J. D. LONDONO, a R. DHARMAPURIKAR, a H. D. COCHRAN, a G. D. WIGNALL, a J. B. MCCLAIN, b. D. E. BETTS, b D. A. CANELAS, b J. M. DESIMONE, b E. T.
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J. Appl. Cryst. (1997). 30, 690-695

The Morphology of Block Copolymer Micelles in Supercritical Carbon Dioxide by Small-Angle Neutron and X-ray Scattering J. D. LONDONO, a R. DHARMAPURIKAR,a H. D. COCHRAN, a G. D. WIGNALL, a J. B. MCCLAIN, b D. E. BETTS, b D. A. CANELAS,b J. M. DESIMONE,b E. T. SAMULSKI,b D. CHILLURA-MARTINOc AND R. TRIOLOc

"Oak Ridge National Laboratory*, Oak Ridge, TN 37831-6393, USA, bDepartment of Chemistry, University of North Carolina, Chapel Hill NC 27599-3290, USA, and CDipartimento di Chimica Fisica, University o f Palermo, 90123 Palermo, Italy. E-mail: [email protected] (Received 16 August 1996; accepted 7 February 1997)

Abstract Above its critical point, carbon dioxide forms a supercritical fluid, which promises to be an environmentally responsible replacement for the organic solvents traditionally used in polymerizations. Many lipophilic polymers such as polystyrene (PS) are insol- uble in CO2, though polymerizations may be accomplished via the use of PS-fluoropolymer stabilizers, which act as emulsifying agents. Small-angle neutron and X-ray scattering have been used to show that these molecules form micelles with a CO2-phobic PS core and a CO2philic fluoropolymer corona. When the PS block was fixed in length and the fluorinated corona block was varied, the number of block copolymer molecules per micelle (six to seven) remained constant. Thus, the coronal block molecular weight exerts negligible influence on the aggregation number, in accordance with the theoretical predictions of Halperin, Tirrell & Lodge [Adv. Polym. Sci. (1992), 100, 31-46]. These observations are relevant to understanding the mechanisms of micellization and solubilization in supercritical fuids.

I. Introduction Small-angle neutron and X-ray scattering (SANS and SAXS) methods allow the elucidation of the size and shape of both individual polymer chains and supramolecular structures (Hayter, 1985; Triolo & Caponetti, 1992;.Wignall, 1987, 1993) in the resolution range 52000 A. Over the past two decades, SANS has emerged as the most powerful technique for studying polymer phase behavior (Wignall, 1987, 1993) and the self assembly of amphiphiles in aqueous media (Hayter, 1985; Triolo & Caponetti, 1992). Breakthroughs in both of these fields were a consequence of the isotropic (H/D) substitution technique in SANS experiments, which marginally affects chemical structure but has a marked effect on SANS intensities. In addition, neutron scattering is particularly suited to studying the structure of © 1997 International Union of Crystallography Printed in Great Britain - all rights reserved

matter under pressure (Nelmes et al., 1993), including supercritical fluids (Londono, Shah, Wignall, Cochran & Bienkowski, 1993). This exceptional capability is granted by the high neutron transmission of many of the materials used in the construction of high-pressure vessels. One other fact that has been used to advantage in the present studies is that fluorinated materials have greater SAXS contrast with the organic core and the solvent. Thus, SAXS and SANS are complementary techniques that highlight different components of the structure, though in the case of SAXS the development of a high-pressure cell is a more challenging task than for SANS. The first experiments to apply these techniques to study polymers in supercritical CO2 have recently been undertaken (McClain, Londono, Romack et al., 1996; McClain, Londono, Chillura-Martino et al., 1996; Chillura-Martino et al., 1997; Fulton et al., 1995) and we have constructed a high-pressure SAXS cell based on the original design of Fulton et al. (Fulton et al., 1995; Camahan et al., 1993). In this paper, we focus on the morphology of micelles formed by polystyrene-bpoly(1,1-dihydroperfluorooctylacrylate) (PS-b-PFOA) copolymers in solution and document the changes in micellar size with variation in solvent density and copolymer concentration, configuration, composition and overall molecular mass. These results constitute an extension and more complete compilation of scattering results on the PS-b-PFOA system than the results published earlier (McClain, Londono, Chillura-Martino et al., 1996).

2. Experimental The procedure used to synthesize the PS-b-PFOA copolymer and the techniques used for its characterization have been reported elsewhere (Canelas, Betts & DeSimone, 1996). Copolymers of seven different compositions were used and are listed in Table 1. Although the polydispersities of the PS blocks (Mw/Mn ~_

Journal of Applied Crystallography ISSN 0021-8898 O 1997

J. D. LONDONO et al. Table 1. M w and M n for the copolymers used Designation PS/PFOA 4/17 4/25 4/40 4/60 4/245 5/25 7/35

M, PS block 3.7k 3.7 k 3.7 k 3.7 k 3.7 k 4.5 k 6.6 k

PFOA block 17k 25 k 40 k 61 k 240 k 25 k 35 k

1.6 + 0.1) are reasonably well known (Canelas, Betts & DeSimone, 1996), those of the PFOA blocks (Mw/M,, ~ 2.5 4-0.5) are less certain. The monomer (segment) masses are 104 g m o l - ~ and 454 g m o l - 1 for PS and PFOA, respectively. The SANS data were collected on the W. C. Koehler 30m SANS facility (Koehler, 1986) at the Oak Ridge National Laboratory (ORNL) via a 64 x 64cm 2 area detector with cell (element) size --, 1 cm 2 and a neutron wavelength, 2 = 4.74 A (A2/2 --, 5%). The detector was placed at various sample-detector distances and the data were corrected for instrumental backgrounds and detector efficiency on a cell-by-cell basis, prior to radial (azimuthal) aver_alging, to give, a Q range of 0.006